This invention is concerned with adding heat to a fluid which is being pumped. The heat serves to increase the temperature of the fluid, or to change its state from liquid to gas, or both. When there is a change of state involved, the process is commonly called vaporization. This can only occur when the pressure at which the fluid is vaporized is below the critical pressure. When the fluid is heated at pressures in excess of the critical pressure, the temperature will always increase, but it is still common to speak of changing the fluid from a liquid to a gas even at supercritical pressures, and this process is also commonly called vaporization. For the purposes of this invention no distinction is made between subcritical and super-critical pressures. When the phrase "heating a fluid to a desired temperature" is used herein, it should be understood that this includes increasing the fluid temperature, or vaporizing the fluid, or any combination of increasing the temperature and vaporizing the fluid so that the desired final fluid temperature and state are achieved.
Systems for pumping and heating a fluid to a desired temperature, as for example heating liquid nitrogen from -320.degree. F. to provide gaseous nitrogen at a desired pressure and temperature, for example 5000 psi and 70.degree. F., are well known in the art. The vaporized nitrogen can be used to displace fluid in oil wells, or for purposes of purging tanks in ships or purging pipelines, or for simply filling nitrogen gas storage bottles.
Heretofore, the known systems usually required burners; direct fired units, boiler systems and the like to effect the heating and/or vaporization. Thus, in addition to an internal combustion engine for driving the cryogenic pump, an additional burner for vaporization is used.
Systems of the foregoing type have certain disadvantages. First, the increased complexity of the system leads to reduced reliability. The operation of the system requires that both the engine and the burner be started and controlled during the liquid pumping and vaporizing process. Experience has shown that systems of this type suffer from field breakdowns caused primarily by inability to start or maintain proper operation of the burner. In contrast to the burner systems, the engines are generally reliable from the standpoint of starting and maintaining controlled operation.
A second disadvantage to the use of burners, particularly of open flame type, is the potential hazard they pose in certain environments where flammable or explosive materials are present.
A third disadvantage of burner systems is that they generally transfer heat from relatively high temperature gases by means of heat exchangers which are prone to failure or "burn out".
There are also known pumping and heating systems which use heat rejected from internal combustion engines such as Otto-cycle engines to vaporize small quantities of fluid in which the work required to pump the fluid is quite small compared to the power rating of the engine. These systems depend on the relatively poor part-load fuel economy of the Otto-cycle engine and the very great disparity between the power available and the power required. They are not practical for pumping and vaporizing significant quantities of liquid.